Measurements of the ductile-brittle transition temperature of heavily doped silicon were carried out using indentation techniques. Diamond pyramid hardness tests were performed on the (100) face of heavily doped N-type and P-type and intrinsic silicon single crystals. Tests were performed over the range 200 C to 850 C and loads of 100 to 500 g were used. Samples were subsequently etched to reveal dislocation rosettes produced by indentation. Intrinsic silicon underwent a ductile-brittle transition at 660 C, P-type at 645 C and N-type at 625 C. Hardness values varied from 1.1 GPa at 700 C to 11.7 GPa at 200 C. Significant effects of hardness on doping were present only at the highest temperatures. Lower loads generally produced higher hardness but load did not affect the Ductile-Brittle Transition Temperature (DBTT). Fracture toughness values ranged from 0.9 MPa m(1/2) at 200 C to 2.75 MPa m(1/2) near the DBTT. Doping did not affect the fracture toughness of silicon. P-type doping increased the size of dislocation rosettes observed after indentation, but N-type did not, in contradiction of the expected results. Results are discussed in terms of the effect of doping on the dislocation mobility in silicon.